[rpc] refactor: consolidate sendmany and sendtoaddress code

The only new behavior is some error codes are changed from -4 to -6.

doc/release-notes-18202.md

@@ -0,0 +1,8 @@
+Low-level RPC Changes
+---------------------
+
+- To make RPC `sendtoaddress` more consistent with `sendmany` the following error
+    `sendtoaddress` codes were changed from `-4` to `-6`:
+  - Insufficient funds
+  - Fee estimation failed
+  - Transaction has too long of a mempool chain